clk: stm32mp1: gets root clocks from fdt

clk: stm32mp1: gets root clocks from fdt

This change makes stm32mp1 clock driver to get the root clocks
reference from the device node in the FDT rather than fetching
straight these clocks by their name. Driver now stores the
clock reference and use it to know if a root clock is present,
get its rate or gets its related udevice reference.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>